Quick Summary

This blog post will delve into the detailed comparison between TypeScript Type vs Interface. Also, we will take a look at the specific use cases of Typescript Interface vs Type to help you gain insight and make the right choice for your next business application development project in 2024.

Table of Contents

Introduction

Comparing TypeScript Type vs Interface, we can infer that both serve different purposes in TypeScript, influencing code clarity and flexibility. While Interfaces define the object shapes, Type Aliases create reusable type declarations. The Type Aliases support the union and intersection types, making them more expressive. Also, TypeAliases can help simplify complex types, improve readability, and more. Understanding the difference between TypeScript Type vs Interface helps developers choose the ideal option for their business application development based on their development needs and requirements.

TypeScript Interface vs Type - Basic Syntax And Extending Types

Basic Syntax - Type Aliases

The Type Aliases are created using the type keyword in TypeScript. They enable your development teams to define custom types by combining the existing types or creating new ones. The syntax for declaring a Type Alias involves mentioning the Alias name and the Type it represents. Generally, it is common to use capital letters for the first letter of the Type Alias name. To make it concise, the developers often use single uppercase letters, such as ‘T’ for generics.

Example – Type Alias

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type TUserProps = {
  name: string;
  age: number;
};

Basic Syntax - Interfaces

Interfaces are defined using the keyword interface in TypeScript. They offer a way to declare custom types precisely for object shapes and contracts. The syntax here involves specifying the interface name and defining the structure of the type. Within Interfaces, it is common to use a capital letter for the first letter of the Interface name. Also, developers often use the ‘I’ as a prefix in the context of class names or object-like structures.

Example – Interface

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interface IUserProps {
  name: string;
  age: number;
}

Extending Types - Type Aliases

Type Aliases use the (&) (ampersand) operator to combine multiple existing types, creating a new type. Referring to the example below, the ‘BusProps’ type is created by intersecting ‘VehicleProps’ and an additional type ‘company’ using ‘&’. This resulted in ‘BusProps’ including all properties from ‘VehicleProps’ and ‘company’.

Example – Type Alias

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type VehicleProps = {
  name: string;
  registrationYear: number;
};
type BusProps = VehicleProps & {
  company: string;
};

Extending Types - Interfaces

The interfaces use the ‘extends’ keyword to declare extension or inheritance, allowing the creation of new interfaces that inherit properties from existing ones. Referring to the example below, the BusProps interface is declared by extending both the VehicleProps and the extra type ‘company’ using the extends keyword. The BusProps interface inherits properties from both the VehicleProps and ‘company’.

Example – Interface

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interface VehicleProps {
  name: string;
  registrationYear: number;
}
interface BusProps extends VehicleProps {
  company: string;
}

Describing Objects and Base Types - Type Aliases vs Interfaces

Comparing TypeScript Interface vs Type, the Type Aliases can help describe a wide range of types, including primitive values (string, number, boolean), allowing for versatility in type definitions.

Example – Describing Primitive Values Using Type Alias

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type Name = string;
const name: Name = "Bacancy";

Interfaces, however, are primarily designed to describe object shapes and are limited to doing so. Therefore, they cannot directly describe the primitive types.

Example – Interface Can Only Describe Objects

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interface Name {
    name: string;
}
const address: Name ={
    name: "Bacancy"
}

Consistency and Readability - Interface vs Type TypeScript

The Type Aliases offer a consistent way to define the types with a simple syntax using the type keyword. This clean and concise syntax improves code readability, making it easier for your development teams to understand and maintain.

Example – Type Alias

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type Employee = {
    id: number;
    name: string;
  };

  type Address = {
    city: string;
    zipCode: string;
  };

  type EmployeeWithAddress = Employee & Address;

  // Usage
  const employee: EmployeeWithAddress = {
    id: 1,
    name: "Bacancy",
    city: "San Diego",
    zipCode: "91945",
  };

Now, comparing TypeScript Type vs Interface, merging interfaces with the same name can lead to unexpected behavior as the properties are combined. The syntax here for the merged or redeclared interfaces sometimes must be clarified precisely when dealing with larger codebases.

Example – Interfaces

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  interface Employee {
    id: number;
    name: string;
  }
  // interface can be redeclared with the same name 
  interface Employee {
    city: string;
    zipCode: string;
  }

  interface EmployeeWithAddress extends Employee {}

  // Usage
  const employee: EmployeeWithAddress = {
    id: 1,
    name: "Bacancy",
    city: "New York",
    zipCode: "10005",
  };

Type Extraction - TypeScript Type vs Interface

Regarding Type Extraction between TypeScript Type vs Interface, the Type Aliases offer a concise and expressive syntax using the typeof operator to extract types from the existing objects easily.

Example – Type Alias

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const product = {
    name: "Laptop",
    details: {
      brand: "Bacancy",
      price: 10000,
    },
    inStock: true,
  };

  // Type Alias using typeof
  type ProductDetails = typeof product.details;

  // Usage
  const laptopDetails: ProductDetails = {
    brand: "Bacancy",
    price: 10000,
  };
  // laptopDetails.color = "Silver"; // Error: Property 'color' does not exist on type 'ProductDetails'.

Interfaces, on the other hand, can quickly achieve similar results, but the syntax within Interface is less concise than Type Aliases as it involves directly specifying the extracted type.

Example – Interface

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// Original Object
const project = {
  title: "Project 1",
  specification: {
    areaSize: 100,
    rooms: 3,
  },
};

// Interface achieving similar results
interface ProjectSpecs {
  areaSize: number;
  rooms: number;
}

// Usage
const specs: ProjectSpecs = {
  areaSize: 100,
  rooms: 3,
};
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Utility Types - TypeScript Type vs Interface

Within the TypeScript Interface vs Type, when using utility types like Omit, the type aliases offer a clean and direct syntax, making the code more readable and expressive.

Example – Type Alias

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type User = {
    id: number;
    name: string;
    age: number;
  };

  // Utility Type: Omit
  type UserWithoutId = Omit;

  // Usage
  const userWithoutId: UserWithoutId = {
    name: "Bacancy",
    age: 23,
  };
  // userWithoutId.id = 123; // Error: Property 'id' does not exist on type 'UserWithoutId'.

Within TypeScript Interface vs Type, the Interface can use the utility types, but it typically requires the extends keyword, resulting in a slightly less elegant syntax than Type Aliases.

Example – Interface

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interface Product {
    id: number;
    name: string;
    price: number;
  }

  // Utility Type: Omit
  interface DiscountedProduct extends Omit {
    discountPercentage: number;
  }

  // Usage
  const discountedItem: DiscountedProduct = {
    name: "Bacancy",
    price: 1000,
    discountPercentage: 10,
  };
  // discountedItem.id = 123; // Error: Property 'id' does not exist on type 'DiscountedProduct'.

Tuples - Typescript Interface vs Type

Regarding describing Tuples, comparing the TypeScript Interface vs Type, the Type Aliases offer a simple and cleaner syntax that helps describe tuples using the square brackets to represent the tuple structure. Considering the example below, the Address type is a Type Alias representing a tuple with elements of types number and string. The syntax within the Type Aliases is concise, using the square bracket to denote the tuple structure.

Example – Type Alias

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type AddressTuple = [number, string, string];

// Utility Type: Omit
type AddressWithoutZip = Omit;

// Usage
const addressWithoutZip: AddressWithoutZip = ["City", "Street"];
// addressWithoutZip[0] = 123; // Error: Type '123' is not assignable to type 'undefined'.

Conversely, the interface can describe tuples but needs a more complex syntax involving readonly properties and explicitly stating the tuple length. In the example below, the Address interface describes a tuple. The readonly and length properties are employed to ensure the tuple structure. The syntax here is more complex and requires explicitly specifying the index positions.

Example – Interface

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interface Address {
  readonly 0: number;
  readonly 1: string;
  length: 2;
}

// Using the tuple interface
const homeAddress: Address = [123, "Main Street"];

Type Alias in Classes

The Type Aliases are well-suited for defining types related to class instances. They offer a clear, concise way to represent and interact with class objects. They are also flexible and consistent when working with various classes or when additional properties must be added to the class interfaces.

Example – Type Aliases in Classes

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type TEmployee = {
  id: number;
  name: string;
};

class Employee implements TEmployee {
  name: string;
  id: number;
  constructor(name: string, id: number) {
    this.name = name;
    this.id = id;
  }
}

TypeScript Type vs Interface When To Use Type Aliases

The Type Aliases offer many advantages over Interface, making them a preferred choice in specific scenarios; let us look at a few of them:

  • Describing Primitive Types: The Type aliases are versatile, allowing them to represent objects and primitive values such as strings, numbers, and booleans. The Interfaces, on the other hand, are mainly designed to shape objects. Therefore, if you work with objects and primitive types, type aliases offer a comprehensive solution.
  • Easier Use of Utility Types: The Type aliases integrate more smoothly with utility types such as Omit. This results in a cleaner and more readable syntax compared to interfaces. Therefore, when utilizing Utility Types to manipulate type in your code, Type aliases offer a more streamlined experience.
  • Consistent Syntax With Other Declarations: The Type Aliases maintain a consistent syntax with other declarations in JavaScript using an equal sign (=) for declaration. This familiarity improves readability, especially for developers who are well-versed in JavaScript. When your codebase follows a consistent syntax, it becomes easier to understand and maintain the code.
  • Predictable Behavior With Merging: The Type Aliases are closed, meaning they cannot be unintentionally redeclared or merged. The feature helps prevent unexpected behavior in larger codebases where multiple developers might work simultaneously. With the Type aliases, you can trust that your type definitions will behave predictably, reducing the chances of bugs and inconsistencies.

TypeScript Type vs Interface - When To Use Interface

The Interface offers several benefits, such as more precise error messages, suitability for specific use cases, the merging feature for dynamic changes, and comparable performance to Type Aliases; let us discuss them in detail:

  • Error Messages with Interfaces: When using interfaces, error messages can be more concise and focused. This is because interfaces define specific contracts that implementations must adhere to, so error messages can be tailored to violations of those contracts.
  • Specific Use Cases for Interfaces: Interfaces are convenient when you need shorter, more targeted error messages. This could be especially useful when you have multiple implementations of an interface and want to ensure each implementation adheres to a specific contract or behavior.
  • Merging Feature of Interfaces: One of the strengths of interfaces is their ability to merge. This means you can extend an interface multiple times in different places, and all the extensions will be combined into a single interface definition. This feature is valuable when developing libraries or when you need to modify or add to interface definitions dynamically.
  • Performance Considerations: Previously, performance was considered when choosing between interfaces and Type Aliases, particularly in larger codebases. However, recent observations suggest that there isn’t a significant TypeScript Type vs Interface Performance difference between the two. Therefore, it might not be a deciding factor when choosing between Interfaces and Type Aliases in modern development.

Conclusion

In conclusion, we can infer that the final choice, when comparing TypeScript Type vs Interface, comes down to your specific project requirements and coding preferences. Both TypeScript Types and Interfaces offer advantages and are exceptional in creating clean, maintainable code. By carefully considering the needs of your project, you can leverage the strengths of each to achieve well-structured TypeScript code. However, if you are a business owner and are still deciding whether to choose an Interface or Type Alias for your project, hire dedicated developers who can offer tailored guidance and expertise to meet your business goals effectively.

Frequently Asked Questions (FAQs)

A type alias in TypeScript allows developers to create a new name for a type. It helps simplify complex type definitions and improve code readability.

Yes, they serve different purposes and can coexist harmoniously in your TypeScript code. Choose the tool that best suits the specific use case for optimal type safety and maintainability.

Yes, both Type Aliases and Interfaces can be used in generic types. This allows for creating reusable and flexible type definitions that can work with different data types and structures.

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